Description: Neuronal circuitry in the spinal cord generates the basic pattern of rhythmic flexion and extension during locomotion. This activity is modified during movement by information about the length of muscles sent to the spinal cord from peripheral receptors. Feedback adapts the spinal motor output to changing conditions and ensures smooth movements. This proposal will locate and identify new classes of spinal cord interneurons activated by group II muscle spindle afferents signaling static muscle length. Despite the powerful effects of the group II reflex system in controlling movements, relatively little is known about the activity of spinal interneurons receiving group Il feedback during locomotion. Electrophysiological recordings will be used to locate and determine the synaptic connectivity of group II responsive interneurons in the lumbo-sacral spinal cord. The spontaneous and evoked activity of these interneurons will be determined during fictive locomotion in decerebrate adult cats. The aims include determining whether mid-lumbar group II interneurons, active during flexion, are inhibitory or excitatory to motoneurons. Stimulation of certain group II afferents promotes flexion while stimulation of group II afferents in other nerves promotes extension. To determine involvement of interneurons in either of these reflex pathways, the activity and responses of interneurons excited by these nerves will be assessed during peripheral nerve stimulation that perturbs the step cycle. Other experiments will examine the involvement of sacral interneurons believed to suppress group II and cutaneous afferent transmission during micturition and locomotion. Anatomical reconstruction of intracellularly stained interneurons will be used to determine the extent of their axonal projections and immunohistochemistry will be used to determine neurotransmitters associated with these interneurons.